This invention relates generally to commutatorless motors, and more particularly to a type thereof wherein induced voltages of a synchronous motor are utilized for detecting the rotor position, and the motor is thereby operated at a predetermined margin angle regardless of variation in stator current.
Known is a commutatorless motor device wherein an angular position of the rotating magnetic poles is detected by a proximity switch or the like, and the commutation of the stator current is controlled by the thus detected position indicative signal. This type of commutatorless motor device, however, cannot compensate for the displacement or shifting of the rotating magnetic field due to an armature reaction, and the power factor, and operational efficiency of the device is deteriorated during the light-load condition, thereby increasing torque ripples.
Recently, there has been developed a control system of commutatorless motors, wherein the position of the rotating magnetic flux is detected directly from the counter electromotive force of the motor, and the stator current is commutated under the control of the thus detected positional signal so as to compensate for any harmful effect of the armature reaction.
In this case, however, since the commutation is of the so-called natural commutation type which is controlled by the counter electromotive force of commutatorless motor, the commutation angle or angle of overlap during the operation of the controlled rectifiers, such as thyristors, is increased in accordance with the increase in the stator current, and a margin angle of commutation is thereby reduced. For this reason, it has been obliged to make large the margin angle under light load conditions so that the commutation advance angle would be excessive. Accordingly, it has been impossible to sufficiently improve the output characteristic of the motor.
As will be described in detail hereinafter with reference to the accompanying drawing, the reduction of the margin angle under a heavily loaded condition of the motor is prevented by beforehand increasing the advance angle of commutation to a great value which is unnecessary during the no-load operation of the motor. In other words, the margin angle of commutation for the control system of the commutatorless motor is varied by the load condition of the motor, thus deteriorating the output characteristics of the motor particularly in its light-load condition.